This invention relates, in general, to electrodeposition, including, but not limited to, electrodeposition of a dense, reflective finish on a conductive part.
Methods of electrodeposition, or plating, of a tin or tin-lead alloy (hereinafter referred to as solder or solder deposit) and the compositions of the electrodeposition solutions have been optimized to electrodeposit solder on to a conductive part. In the electronics industry, a conductive part could be the leads of a semiconductor device package, a printed circuit board, or connector.
In particular, in the manufacture of semiconductor devices, the semiconductor device chip is physically and electrically bonded to a leadframe. The semiconductor device is then encapsulated in a package, along with a portion of the leadframe. An electrodeposition process then creates a solder deposit on the leadframe by electrodepositing the solder on all exposed portions of the leadframe.
Following the electrodeposition process, a trim and form press or tool trims away all unwanted metal from the leadframe, singulates the devices, and forms the leads of the device into a predetermined pattern. In the electronics industry it is preferable that the solder deposit have a dense, reflective finish.
The dense, reflective finish is preferable for quality reasons. The higher density and smoothness of a dense, reflective finish reduces the amount of material scraped from the surface of the deposit during the trim and form operations. Scraped material from a normal, matte finish contaminates subsequently processed leads by adhering to the surface of such leads. If a dense, reflective surface is deposited, the need to clean trim and form tools is reduced because the amount of material scraped from the surface of the solder deposit is reduced, and thus productivity is enhanced.
In the past, one problem with electrodepositing a tin or tin-lead alloy having a dense, reflective finish is that such deposits have 800-2000 ppm (parts per million) of occluded carbon (organics). The co-electrodeposition of carbon is not a problem in certain applications. However, in the electronics field, greater than approximately 500 ppm of carbon co-electrodeposited with the tin or tin-lead alloy negatively affects the solderability of the deposit. Therefore, it is desirable to have a method of electrodeposition (and/or use electrodeposition solutions) which produces a dense, reflective tin or tin-lead alloy finish without the co-electrodeposition of greater than approximately 500 ppm of carbon.